FSK transmitters and receivers in combination to form a modem for receiving and transmitting FSK signals are well known in the art. However, in these prior art systems, the majority of the receiver gain is at R.F. or I.F. frequencies where costly multipoled crystal filters must be used and in addition, tracking R.F. narrowband tuned bandpass filters are required to attenuate spurious noise generated in mixers. In addition, mark and space channel amplifiers must be accurately gain and phase-matched. Very accurate frequencies must be generated by crystal or SAW devices in order to provide proper receiver selectivity. Because mixers are used to generate I.F. and lower frequencies, spurious responses are generated which requires the need for complex R.F. filtering. In a like manner, in the modulator-transmitter, modulation at intermediate frequencies requires translation mixers and tuned R.F. filters which are required to attenuate spurious noise generated by the mixers.
The present invention overcomes the disadvantages of the prior art by providing a full duplex FSK system in which the modulator and demodulator are extremely broadband. Center frequency operation is controlled by a synthesizer injection frequency. Further, a single frequency synthesizer provides the R.F. signals required for both receiver and transmitter functions. Translation of the frequency-shift-keying R.F. spectrum directly to baseband in the receiver drastically reduces hardware complexity without sacrificing performance. Thus, the majority of the receiver gain can be at baseband where gain elements are high performance and relatively simplistic and inexpensive. DC response is not required and mark/space channel amplifiers do not have to be accurately gain and phase-matched. Receiver selectivity is provided by baseband L/C components or active filters. Crystal or SAW devices are not required. Adequate receiver dynamic range is achieved by the use of automatic gain control. The level detectors and differential output stage have an additional 30 dB linear range. Because receiver signals are converted to baseband (with no intermediate frequency) many spurious responses are eliminated. This reduces the need for complex R.F. filtering. The receiver section also uses an image-cancellation mixer wherein sum and difference signals generated by a pair of double-balanced mixers are added together with proper phase to cancel out the image response normally associated with heterodyne receivers.
The transmitter section of the FSK system includes an FSK modulator whose peak deviation is controlled by an accurate derivative of a crystal clock. The FSK modulator functions in the manner of a phasing single sideband mixer where upper or lower sideband signals are generated by the proper phasing of quadrature components of the carrier frequency and the peak deviation signal derived from the crystal clock. Again, translation of the frequency-shift-keying R.F. spectrum directly from baseband drastically reduces hardware complexities without sacrificing performance. Such translation directly at final frequency eliminates translation mixers and the tuned R.F. filters required to attenuate spurious noise generated by mixers.
Thus, the present invention provides a full duplex FSK receiving and transmitting system.
The invention also provides a full duplex FSK receiving and transmitting system in which the receiver section of the system uses an image-cancellation mixer to cancel out the image response normally associated with heterodyned receivers.
The present invention provides a full duplex FSK receiving and transmitting system in which translation of the frequency-shift-keying R.F. spectrum is accomplished directly to and from baseband frequency.
The invention additionally provides a full duplex FSK receiving and transmitting system in which the modulator in the transmitter is similar to a phasing single sideband mixer where upper or lower sideband signals are generated by the proper phasing of quadrature components of the center frequency and the deviation frequency signals.
The invention also provides an FSK system in which the majority of the receiver gain can be at baseband where gain elements are high performance and relatively simplistic and inexpensive.
Also in accordance with the present invention, there is provided an FSK system in which the receiver signals are converted directly to baseband without any intermediate frequency, thus, eliminating spurious responses and the need for complex R.F. filtering.
The present invention also provides an FSK system wherein modulation takes place directly at final frequency to eliminate the translation mixers and tuned R.F. filters required to attenuate spurious noise generated by mixers.